1. Field of the Invention
The present invention relates to a secondary sealed battery in which safety features are obtained by reducing internal pressure and cutting-off current flow when the internal pressure of the battery is increased above an allowable level by an abnormal operation of the battery.
2. Description of the Related Art
Generally, a sealed battery has the drawback that it may explode when internal pressure is increased above an allowable level as a result of overcharging or the abnormal operation thereof.
U.S. Pat. No. 5,418,082 discloses a sealed battery with a circuit breaker that can prevent such battery explosions.
FIG. 11 shows a sealed battery disclosed in the U.S. Pat. No. 5,418,082.
A sealed battery comprises a can 2 in which an electric generator 4 is inserted together with electrolyte. A safety valve 6 is air-tightly mounted on an opened top of the can 2 with a gasket 8 disposed therebetween. A lead 12 is disposed on an upper surface of the safety valve 6. The safety valve 6 is provided with a projection 6a. The projection 6a is projected toward the electric generator 4.
Disposed between the safety valve 6 and the generator 4 is a disk 14 provided with a center hole 14a, into which the projection 6a is inserted, and a gas hole 14b. Disposed on a bottom surface of the disk 14 is a metal membrane 16 contacting a lead 18 of the electric generator 4. A disk holder 20 is disposed between the safety valve 6 and the disk 14.
For use as a circuit breaker 10, the projection 6a of the safety valve 6 and the metal membrane 16 are welded to each other. When internal pressure of the battery is increased above an allowable level, the safety valve 6 is deformed by the increased internal pressure such that the projection 6a of the safety valve 6 and the metal membrane 16 that are welded to each other are separated, thereby preventing the chance of explosion.
The severing force of the circuit breaker 10 is determined by the strength of the weld between the projection 6a and the metal membrane 16. However, since the weld strength is determined by a variety of parameters such as power, energy, amplitude, pressure of a welding apparatus, welding time, a welding gap, and material used in the welding process, it is difficult to maintain a uniform weld strength for each circuit breaker 10. As a result, the severing force of the circuit breaker 10 is varied for different circuit breakers 10.
In addition, it is possible for the weld between the safety valve 6 and the metal membrane 16 to oxidize by reacting with the electrolyte with the passage of time such that the strength of the weld force is reduced. Accordingly, the safety valve 6 and the metal membrane 16 welded to each other may be separated even when the internal pressure is not increased, causing the malfunction of the battery.
Therefore, the present invention has been made in an effort to solve the above described problems.
It is an objective of the present invention to provide a sealed battery having a circuit breaker which accurately operates at a predetermined internal pressure of the battery.
To achieve the above objective, the present invention provides a sealed battery comprising a can for receiving an electric generator, the can having an opened top end, a deformable plate air tightly mounted on the opened top end with a gasket disposed between the deformable plate and an inner wall of the opened top end, a terminal cap disposed on the deformable plate, the terminal cap being elevated except for an edge, and a circuit breaker disposed between the deformable plate and the terminal cap to cut-off current flow when the deformable plate is deformed by internal pressure increased above an allowable level, wherein the circuit breaker comprises an insulating plate having a bridge disposed traversing the deformable plate and provided with a mechanically weak portion and a via hole, the mechanically weak portion being severed when the deformable plate is deformed by internal pressure increased above a predetermined level, an upper conductive layer formed on an upper surface of the insulating plate, and a terminal member disposed through the via hole to electrically connect the upper conductive layer to the deformable plate.
The circuit breaker further comprises a lower conductive layer formed on a lower surface of the insulating plate, the terminal member being connected to the deformable plate through the lower conductive layer.
The upper conductive layer comprises a semi-bridge having a length less than that of the bridge such that the upper conductive layer has a first end located on one end of the bridge and a second end located on a portion of the bridge where the second end does not a contact the terminal cap.
Alternatively, the lower conductive layer comprises a semi-bridge having a length less than that of the bridge such that the lower conductive layer has a first end located on the other end of the bridge and a second end located on a portion of the bridge where the second end does not contact the terminal cap, the second end of the upper conductive layer being overlapped by the second end of the lower conductive layer.
Each of the upper and lower conductive layers and the insulating plate comprises a ring-shaped member.
The mechanically weak portion is defined by a hole formed on a middle portion.
The hole formed on the middle portion functions as the via hole. In addition, additional holes are formed on portions in the vicinity of opposite ends of the bridges to provide additional mechanically weak portions.
The upper and lower conductive layers are made of copper or solder.
The current breaker further comprises insulating layers respectively deposited on the upper and lower conductive layers except for a portion of the upper conductive layer which contacts the terminal cap, and a portion of the lower conductive layer which contacts the deformable plate. The insulating layer is deposited using insulating ink.
The deformable plate is provided with a safety groove which can be broken by a predetermined internal pressure of the battery. The predetermined internal pressure for breaking the safety groove is higher than the internal pressure for severing the mechanically weak portion.
A displacement of the deformable plate, which occurs by internal pressure increased above the predetermined allowable level, is set less than 1 mm.
The deformable plate is flat and comprises a reinforcing bead.
The sealed battery of the present invention may further comprise a current control member disposed between the circuit breaker and the terminal cap to cut-off current flow when a temperature of the battery is increased above an allowable level.
The current control member comprises upper and lower metal plates and a mixture layer disposed between the upper and lower metal plates, the mixture layer being made of carbon and polymer and the lower metal plate being integrally formed with the upper conductive layer of the circuit breaker.